Thanks to Amanda, who forwarded me this set of photos featuring the denning behavior of rattlesnakes. Forwards are of course not always to be trusted, but this one claims the dens are in the area of Fort Smith, Arkansas. Rattlesnakes congregate for the winter, sometimes by the thousands. These look to me like prairie rattlers, the kind I mostly grew up with in Oklahoma. For more about rattlesnake denning, see The Red Hourglass.